Polarized continuum emission from dust grains has been detected in a wide variety of Galactic sources. It is widely believed that the local magnetic field acts to align elongated grains, so that they spin with their long axes perpendicular to the field. This results in linear polarization, which can be used to measure the direction of the field component in the plane of the sky. The polarization position angle, , is perpendicular to the net field within the area observed. The percentage polarization p is not so generally useful, as it is a function of several parameters, including grain alignment efficiency, optical depth, inclination of the field to the plane of the sky, and the amount of local disorder in the field. However, the value of p sometimes correlates with other source properties (e.g. in protostars, Greaves et al. 1997), and has also been used to constrain grain alignment efficiencies (Roberge 1996, Hildebrand & Dragovan 1997).
Although many data have been obtained on the polarization of dust emission, it is still unclear exactly how the grains are aligned (Hildebrand 1996). Paramagnetic relaxation (Davis & Greenstein 1951) was favoured, but it is now known that this mechanism aligns small grains most efficiently, the opposite to what is observed (e.g. Lazarian 1996a), and that unrealistically large gas and dust temperatures differences are required for molecular clouds (Roberge 1996). Alternatives include superparamagnetism (paramagnetism enhanced by iron compound inclusions in the grains, e.g. Sorrell 1994); mechanical alignment by collisions with gas molecules (Roberge et al. 1995); and radiative alignment where a torque is induced on asymmetrical grains (Draine & Weingartner 1996, 1997). A necessary condition for many of the alignment models is that the grains spin at suprathermal speeds. This spin-up could be due to radiative torque (Draine & Weingartner 1996), or initiated by cosmic rays which release H2 molecules from grain mantles providing a `rocket effect' (Sorrell 1995a). Finally, it has been suggested by Onaka (1995) that polarization can arise without alignment, if grain heating depends on the cross-section presented to an illuminating source. This mechanism can also affect the polarization if the grains are aligned (Onaka 1996).
We have made polarization observations at 800 and 1100 of three star-forming molecular clouds, including Mon R2, W3 and DR21. The results are discussed here, and used to constrain the above models for the production of submillimetre polarization in dust clouds.
© European Southern Observatory (ESO) 1999
Online publication: March 18, 1999